Method and means for damping supersynchronous oscillations in an ac power system

ABSTRACT

Damping of supersynchronous oscillations in an ac power system is accomplished by a damping circuit connected between a power line and a system neutral potential. The damping means includes a tuned circuit having a resonant frequency above the system operating frequency. Preferably, the tuned circuit further has an antiresonant frequency at the system frequency. The supersynchronous damping circuit in accordance with the invention can be used with subsynchronous damping circuits known in the prior art to enhance the damping of oscillations in the ac power system.

This invention relates generally to electric power systems, and moreparticularly the invention relates to a method and means for eliminatingvoltage and current supersynchronous oscillations in ac power systems.

Alternating current (AC) electric power systems are normally operated at50 Hz or 60 Hz. Since a plurality of power sources can be connected to apower system, and further since independent power systems are normallyinterconnected with one another for back-up power sharing purposes, themaintenance of an exact frequency for the current and voltage in thepower system is imperative. Voltage and current oscillations at otherfrequencies can create serious problems of stability of the system andsafety of equipment in the system. These oscillations can be started bysmall or large disturbances and become a problem when system damping islow or even regenerative at certain frequencies.

For example, when generators are connected to a power system or when twopower systems are connected by transmission lines, oscillations in thefrequency range of 0.1 Hz to 5 Hz may be present. Such oscillationscause system instability.

Further, when large turbine-generator units are connected to a powersystem through long series-compensated transmission lines, oscillationsin the range of 10-50 Hz often occur due to interaction of mechanicaloscillations of a long turbine-generator shaft and the electricalsystem. Such oscillations can damage the generator shaft even when theamplitude thereof is small.

Alternatively, when a transmission line near a generating plant isdisconnected to clear a fault, it is desirable to restore the lineconnection within a few cycles by high-speed reclosing of the breakers.This, in many applications, is not permitted because the oscillations ofthe machine started by the sudden change in load take a long time todecay, and mechanical oscillations caused by reclosure action could addto the earlier oscillations on the machines, and the resulting totaltorque and shaft twisting may exceed the acceptable limits. If, however,the oscillations could be made to damp more quickly, then fasterreclosure of the disconnected line would be possible.

Disclosed in U.S. Pat. No. 4,434,376 is means for negatively dampingsubsynchronous oscillations and dc offset is an ac power system. Thedamping apparatus may be either series connected or shunt connected inthe power system. The series connected apparatus includes reactancemeans serially connected in a power line and circuit means connected inparallel with reactance means including a resistor and seriallyconnected switch means. Means is provided for triggering switch meanswhen half cycle time periods of the voltage across reactance means inthe power line exceed a desired half-cycle time period.

A shunt connected apparatus is disclosed also in which circuit means isprovided for connecting a power line to a circuit neutral terminal orground including a resistive means and a serially connected switchmeans.

Again, means is provided for triggering the switch means in response tohalf cycle time periods of the voltage waves on the power line exceedinga desired half cycle time period.

Thus, the embodiments disclosed in the patent are concerned with dampingsubsynchronous oscillations and not specifically with supersynchronousoscillations or frequencies greater than the line operating frequencysuch as 60 Hz.

Accordingly, an object of the present invention is improved method andmeans for damping supersynchronous oscillations in a power system.

Briefly, a reactance network is provided in parallel with the electricgenerator. The reactance network is tuned to a supersynchronousfrequency to provide positive damping. In a preferred embodiment, thereactance network is antiresonant at the system operating frequency andthereby tends to block current at the operating frequency whileproviding a low impedance path for supersynchronous currents. Aplurality of reactance networks can be provided which are tuned todifferent supersynchronous frequencies.

Advantageously, the positive damping of the supersynchronous network inaccordance with the invention can be combined with the subsynchronousnetwork which provides negative damping as taught in U.S. Pat. No.4,434,376.

The invention and objects and features thereof will be more readilyapparent from the following detailed description and appended claimswhen taken with the drawings, in which:

FIG. 1 and FIG. 2 are functional block diagrams of an electrical powersystem including damping apparatus in accordance with the prior art.

FIG. 3 and FIG. 4 are electrical schematics of two embodiments ofsubsynchronous damping apparatus in accordance with the prior art.

FIG. 5 is a functional block diagram of an electrical power system inaccordance with the present invention.

FIG. 6 and FIG. 7 are electrical schematics of two embodiments ofsupersynchronous damping networks in accordance with the invention.

Referring now to the drawings, FIGS. 1 and 2 are functional blockdiagrams of electrical power systems including subsynchronous dampingapparatus in accordance with the prior art. A generator 20 is connectedto a power system transmission system 22 through a series connecteddamping circuit 24 in FIG. 1. The damping circuit may be near the pointof connection of the generator or may be along the transmission lines.It is common practice to provide series capacitor compensation atseveral locations along the line routes, and the damping circuit may beprovided at one or more of these capacitor compensations. It is assumedthat the power system transmission line carries voltage at 60 Hz, andthe generators generating a 60 Hz voltage with a spurious oscillationgenerating a subsynchronous frequency component, delta Hz.

FIG. 2 is a block diagram illustrating a generator 21 connected to apower transmission system 23 and a shunt connected damping circuit 25for damping subsynchronous oscillations. The shunt damping circuit maybe near the point of connection of the generator or may be along thetransmission lines.

The damping networks for use in the power systems of FIGS. 1 and 2 aredesigned for negatively damping subsynchronous oscillations. In FIG. 3capacitor 28 is serially connected with transmission line 30 astypically employed to compensate for line inductance. Dischargingcircuit means is connected in parallel with capacitor 28 for dischargingthe excess voltage wave periods and includes a resistor 32 seriallyconnected with thyristors 34 and 36. The non-linear resistor 35 can beconnected in parallel with thyristors 34, 36 as shown. The thyristorsare connected in parallel and in opposite polarity whereby current flowthrough resistor 32 can be selectively controlled in either direction.When high voltage is involved the thyristor may be replaced by anassembly of thyristors which together act as a thyristor. Thyristor 34is triggered conductive by a control signal when the excessive capacitorvoltage half cycle period is positive, and thyristor 36 is triggered fora control signal when the excessive capacitor voltage half cycle isnegative. In either case, after the larger half cycle is detected, thecorresponding thyristor is fired to discharge the capacitor throughresistor 32 for the excess portion of its half cycle voltage. Thisconduction causes capacitor 28 to discharge and accelerate the currentzero of the capacitor voltage.

In FIG. 4 the subsynchronous damping network is shunt connected to thepower system. In this embodiment the damping circuit including resistor32 and thyristors 34 and 36 are connected between the transmission line30 and a system neutral terminal such as ground. The damping circuitsoperate in the same manner as the damping circuit of FIG. 3; however, inthis embodiment the voltage of the transmission line is monitored ratherthan the voltage across the series capacitor. When a half cycle of theline voltage is too large, the damping circuit is rendered conductive byapplying the suitable trigger circuit to either thyristor 34 orthyristor 36, depending on the polarity of the line voltage, and theexcess voltage is shunted to ground through resistor 32. Thus, the linevoltage half cycles which are too long are forced towards zero levelwith consequent damping of the subsynchronous oscillation.

Consider now FIG. 5 which is a functional block diagram of an electricalpower system which includes a subsynchronous network as described aboveand a supersynchronous network in accordance with the present invention.The subsynchronous network 40 is connected in parallel with capacitor 42which is serially connected in the transmission line. A supersynchronousnetwork 44 shunts the generator 46 to ground. Accordingly, anysubsynchronous oscillations or dc offset can be dampened by thesubsynchronous network 40 whereas any supersynchronous frequencies aredamped by the network 44.

FIG. 6 is an electrical schematic of a supersynchronous network inaccordance with one embodiment of the invention. In this embodiment afirst inductor L2 is serially connected with capacitor C1 and inductorL1 which are connected in parallel. The inductors and capacitors areconnected to ground by a switch 50 which may comprise parallelthyristors as in FIGS. 1 and 2. In this embodiment the parallelcomponents C1 and L1 are tuned to provide an antiresonant circuit at theoperating frequency of the power system, such as 60 Hz. The seriallyconnected components are tuned to a supersynchronous frequency, such as80 Hz, for damping supersynchronous frequencies when switch 50 isclosed.

FIG. 7 is another embodiment of the supersynchronous damping networkwhich includes two supersynchronous paths tuned to different frequenciesabove the system operating frequency. The inductors LM are mutuallyinductively coupled to reduce or eliminate circuit interactions, and theserially connected inductors and capacitors C1 and C2 are tuned todifferent supersynchronous frequencies, such as 80 Hz and 100 Hz. Again,a switch 52 is provided for connecting the supersynchronous dampingnetwork to circuit ground in response to the detection ofsuperynchronous frequency waves in the power system.

The provision of positive damping to supersynchronous frequency waves ina power transmission system in accordance with the invention enhancesthe operation of electric power transmission systems when combined withthe subsynchronous networks of the prior art. While the invention hasbeen described with reference to specific embodiments, the descriptionis illustrative of the invention and is not to be construed as limitingthe invention. Various modifications and applications may occur to thoseskilled in the art without departing from the true spirit and scope ofthe invention as defined by the appended claims.

What is claimed is:
 1. Apparatus for damping supersynchronousoscillations in an AC power line operating at a system frequencycomprising a parallel tuned circuit and switch means for serialinterconnection between said power line and a system neutral potential,said tuned circuit havint a resonant frequency above said systemfrequency and an antiresonant frequency at said system frequency, saidtuned circuit including first inductive means serially connected withfirst capacitive means, second inductive means serially connected withsecond capacitive means, said first inductive means and said firstcapacitive means being connected in parallel with said second inductivemeans and said second capacitive means, said first inductive means andsaid second inductive means including mutually coupled inductors. 2.Apparatus as defined by claim 1 wherein said first inductive means andsaid first capacitive means are tuned to a first supersynchronousfrequency, and said second inductive means and said second capacitivemeans being tuned to a second supersynchronous frequency.
 3. Apparatusfor damping oscillations in an AC power line operating at a systemfrequency comprisinga first damping circuit connected in series withsaid power line including capacitive means serially connected in saidpower line and switch means for selectively discharging said capacitivemeans, a second damping circuit connected between said power line and asystem neutral potential, said second damping circuit including switchmeans and a tuned circuit having a resonant frequency above said systemfrequency, said tuned circuit of said second damping means includingfirst inductive means serially connected with first capacitive means,second inductive means serially connected with second capacitive means,said first inductive means and said first capacitive means connected inparallel with said second inductive means and said second capacitivemeans, said first inductive means and said second inductive meansincluding mutually coupled inductors, said first inductive means andsaid first capacitive means being tuned to a first supersynchronousfrequency, and said second inductance means and said second capacitivemeans being tuned to a second supersynchronous frequency, and anantiresonant frequency at said system frequency.